Marking the passing of time

We are constantly tracking the passage of time, from waiting for our kettle to boil to monitoring traffic lights. The ways in which we perceive time usually depend on the timescale involved, ranging from minutes to hours to days. Interestingly, it seems that we and other animals can estimate the duration of very small intervals of time with remarkable accuracy. But how does the brain do this? This is the question explored in a recent PLOS Biology article by Blaine Schneider and Geoffrey Ghose, which is also discussed in an accompanying Primer by Eric Cook and Christopher Pack.

Neuroscientists and psychologists have long been interested in studying the perception of time, because accurate timing serves as the basis for many behaviors, from foraging strategies in birds to performing music. From an evolutionary standpoint, one can imagine that as a prey, it’s important to gauge how long you have before your predator becomes hungry again. And in a more “real-world” example, you could also imagine why it’s important to have an accurate internal sense of time when trying to cross a busy street. You have to use visual information to estimate motion trajectories, but you also have to use past experiences (like how fast you can run!) to estimate your future position.

As Cook and Pack discuss, however, the picture becomes altogether more complicated when our brains attempt to estimate time over much smaller time scales. This process involves multiple brain areas – including the parietal cortex and cerebellum, to name but two – all of which display neural activity correlated with time perception.

Many recent studies of non-human primates have found neuronal activity that correlates with time perception in a region of the parietal cortex called LIP. Traditionally, LIP activity has been linked to the generation of eye movements called saccades. These studies found that LIP activity increases when an animal is waiting to saccade and further increases just before the actual saccade.

A previous study of time perception by Matthew Leon and Michael Shadlen used a task in which monkeys had to make a saccade based on their own measurement of time. These researchers confirmed that LIP activity correlated with the saccade, as previous work had shown, but they were also able to show that LIP activity increased steadily in trials of a longer duration. Combined with other results, Leon and Shadlen concluded that LIP activity is related to internal time perception, and that increases in LIP activity may be the neural substrate for how the brain measures time.

But is this the whole story? As explained in the Primer by Cook and Pack, there are some interesting caveats here that Schneider and Ghose explored in their PLOS Biology paper. One important aspect of the task designed by Leon and Shadlen was that saccades were always associated with a reward. So what Schneider and Ghose did in this study was to eliminate external cues from their task design, including the correlation between saccades and rewards. They did this by training monkeys to move their eyes consistently at regular time intervals without any external cueing or expectation of an immediate reward.

Surprisingly, when they used this task, they found the opposite result to Leon and Shadlen. LIP activity actually decreased at a steady rate when monkeys prepared to make saccades. Clearly, some aspect of their new task design changes how LIP activity responds during saccade timing. The predictability and anticipation of the reward is one of the key differences, and an interesting hypothesis to explore in future studies.

So why do our brains perceive the passing of small time intervals, at the level of seconds or even subseconds? One possible reason is that it helps us to integrate information over these small time scales, allowing us to evaluate how sensory inputs change multiple times a second and to keep track of our own movements. Whether LIP activity is causally related to this process, and how other neural circuits are involved, are still open questions. But given the philosophical implications for how we measure time, and how this relates to our perception of “free will”, there’s no doubt we have some exciting results to look forward to from this field.